1. Field of the Invention
The present invention relates to thin film magnetic memory devices, and more particularly to a thin film magnetic memory device provided with memory cells having magnetic tunnel junctions (MTJ) (hereinafter, also simply referred to as the “MTJ memory cells”).
2. Description of the Background Art
In recent years, a magnetic random access memory (MRAM) device has attracted attention as a next-generation nonvolatile memory device. The MRAM device stores data in a non-volatile manner using a plurality of thin film magnetic elements formed in a semiconductor integrated circuit, and permits random access to the respective thin film magnetic element.
In particular, recent announcement shows that the performance of the MRAM device is significantly improved by using thin film magnetic elements utilizing the magnetic tunnel junctions as memory cells.
It is common in memory devices to perform normal operations of reading and writing data and test operations based on program information stored therein in a non-volatile manner. Typical program information for the normal operations includes one used for redundant configurations for replacement of defective memory cells with spare memory cells provided additionally.
For the test operations, typically, data used for a so-called built-in self-test (BIST) for internal testing of data write/read is stored as the program information. U.S. Pat. No. 6,347,056 B1 discloses a configuration to store program information for use in the BIST using MTJ memory cells.
In a conventional memory device, program information would be programmed by blowing fuse elements with laser or the like. As the configuration required special equipment such as a trimming device dedicated to laser blowing, enormous time and cost were spent for the programming process. In addition, such programming is commonly performed at a wafer level. Thus, once a memory device is packaged as a final product after defective addresses corresponding to defective memory cells detected at the wafer level have been programmed, it would be difficult to remedy any defects that occur to the packaged product, resulting in degradation of the yield.
Further, to program a large volume of program information, correspondingly a large number of fuse elements and others must be provided, leading to an increased area of circuit band for storage of the relevant program information.
The MTJ memory cell described above is advantageous in that it is capable of rewritable, nonvolatile data storage, and ensures a high degree of integration, so that a large volume of program information can be stored in a small area. U.S. Pat. No. 6,324,093 B1 discloses a configuration to perform nonvolatile data storage by destroying magnetic memory elements of the MTJ memory cells. Japanese Patent Laying-Open No. 2001-217398 discloses a configuration to store a large amount of information in a small area by storing multi-valued information in an area of one memory cell.
The MTJ memory cell, however, is disadvantageous in that, since it is a memory element utilizing the characteristic of thin film magnetic element that its electric resistance is changed according to a magnetization direction, data once programmed would be rewritten, or the program information would be lost, due to magnetic noise and others.